Works with interrupts

Dependents:   Nucleo_UltrasonicHelloWorld Proj Nucleo_UltrasonicHelloWorld mbedTurtleCar ... more

Fix in the constructor without onUpdate function.

The constructor without onUpdate function specified does not work properly. The pointer to the callback function is not initialised, and the timer is not started properly. These changes seem fix this issue.

Updated version of the library to deal with the constructor without onUpdate callback function.